“…This is motivated by the fact that many crystals appearing in nature belong to this class, for example, all metals of cubic symmetry, Au, Fe, Ni, Ag, and ionic crystals such as, for example, NaCl, KCl, LiF, AgCl, and many more. Let us consider a cubic crystal of class m3m in which the strain energy W is defined by three second-order and six third-order elastic constants [5]: c 12 , c 44 , c 111 , c 112 , c 144 , c 123 , c 166 , c 456 ).…”